Why do I dream about him very often?

The reason you dream about him is because you continuously thinks about him. even you think about hims before falling asleep, he is on your mind, you care about him, you may have hidden feelings, he is the only thought on your mind.

Dreams are weird things. They don't generally mean very much, but they are a reflection of what our brains need to store about things we learned and thought about during our waking hours. The individual elements don't tie together in reality the way they do in dreams (ie. If you dream that your sister is a train, it says nothing about the way you see your sister, just that you are storing something about your sister and also storing something about trains). Patterns are what matters. So, if you are dreaming about someone a lot your brain is storing information about that person, or about things that it relates to that person.

It depends on what kind of dream it is. If it's a good dream then it usually means that you like him, and if it's a bad dream it usually means that you don't and that he frightens you in some way.